Private-branch-exchange circuits



NOV, 19240 R. S. BAILEY PRIVATE BRANCH EXCHANGE CIRCUITS Filed 001?. 251920 INVENTOR ATTORNEY Patented Nov. 11, 1924.

UNITE. STE.

Maia

RAND S. BAILEY, 0F MONTCLAIR, lbTEiV JERSEY, ASSIGNOR T0 AMERICANTELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

PRIVATEsBRANGH-EXCHANGE CIRCUITS.

Application filled October 25, 1920.

1/ 0 all 107mm it may 0071 (36771 Be it known that I, RAND S. BAILEY,residing at Montclair, in the county of Essex and State of New Jersey,have invented certainv Improvements in Private-Branch-Ex changeCircuits, of which the following is a specification.

This invention relates to telephone systems, and more particularly to alink circuit arrangement for establishing connections betweensubscribers.

One of the objects of the invention is to provide a link circuitarrangement for estal'ilishing connections between the extension linesof a private branch exchange, and between such extension lines andtrunks leading to the main central ofiice.

Another object of the invention is to provide a link circuit of thecharacter just described, which is ad.apted to the requirements of aprivate branch exchange associated with the main exchange in which connections are established by mechanical switches.

Another object of the invention is to provide a link circuit ofthe'character just described, which is so arranged that in theestablishment of connections between extension lines and trunk linesleading to the main exchange, through supervision will be provided afterthe connection to the trunk line is established so that the subscribersupon the extension line will have control of the supervisory apparatusat the main central exchange as well as the supervisory apparatus at theprivate branch exchange.

Another object of the invention is to provide a link circuit or cordcircuit of the character above described in which, when a subscriber atthe private branch exchange hangs up after being connected to, a trunkline leading to the central oliice, the trunk line will automatically befree for an incoming call from the main otiice.

Another object of the invention is to prw vide a cord circuit of suchcharacter that under the conditions set forth in the precedingparagraph, the cord circuit will be opened to prevent a ringing signalincoming from the main otlice over the trunk line from ringing the bellof the subscriber at the private branch exchange in case the connectionhas not been taken down.

Another object of the invention is to pro vide means whereby under thecircumstances Serial No. 419,389.

just. described, the subscriber on an extension line of the privatebranch exchange may recall the operator at the private branch exchangein order to establish another call, or

may initiate a new call at the distant central oiiice without the aid orintervention of the private branch exchange operator.

Another object of the invention is to provide means whereby, under theconditions previously described, a recall may be made over the trunkline from the main central oliice.

Another object of the invention is to pro- .vide means in connectionwith the cord cirby the subscriber on the extension line.

Still another object of the invention is to provide means whereby, whenthe operators set has been connected to the cord circuit during thedialing operation, the subscribers may carry on a conversation throughthe operators set after the connection has been completed in case theoperator should for any reason, fail to, disconnect her set.

A further object of the invention is" to provide means wherebv the cordcircuit may be used during periods when no operator is in attendance atthe private branch exchange for patching a connection from the trunkline leading to the main exchange directly to an extension line of theprivate branch exchange so that an attendant at the station associatedwith the extension line may answer incoming calls from the mainexchange.

These objects, as well as other objects of the invention, may beattained by means of the apparatus set forth in the followingdescription and illustrated in the accompanying drawing, the figure ofwhich illustrates a circuit diagram of one embodiment of the invention.

Referring to the drawing, a cord circuit C for establishing connectionsat a private branch exchange is shown terminating in plugs P, and P Theplug P is adapted lit) to co-o'perate with the jack J, of a trunk line Lleading to the main central oflice, or with jacks such as J of extensionlines such as IE leading to individual stations of the private branchexchange. The plug 1? is also adapted to co-operate with jacks such as Jof extension lines such as E extending to individual stations of theprivate branch exchange. The trunk line L may terminate at a maincentral ofiice at which connections are established either manually orby means of switching machinery. Ringing keys K and K are associatedwith the front and back' cords terminating in the plugs P, and Prespectively, for ringing the stations connected to the private branchexchange when local connections are established. A key K is alsoProvided for associating the operators set with the cord circuit, thekey being so arranged that when operated the conductors of the front andback cords, instead of being directly connected together, areinterconnected through the operators set, the operators set beingprovided with the usual transmitter and receiver R, and in addition, adial D is provided in order that the operator may establish connectionsby means of the switching machinery at the main central oflice. in thecord circuit C, this key being for the purpose of connecting the frontand back cords of the cord circuit independently of the supervisoryapparatus. When this key is thrown and the cord is used for connectingan extension line to the trunk line leadv ing to the central oflice, thesubscriber on the extension line may dial directly through the cordto'set the switching machinery at the main oflice. The cord may also beused in this condition for connecting the trunk line leading to the mainofiice to some individual extension line so that during periods when nooperator is in attendance at the private branch exchange, the trunk lineis, in effect, extended directly to a subscribers station.

Further details of the apparatus will now be clear from the descriptionof the operation, which is as follows:

Assuming that a subscriber at the main central oflice desires to beconnected to a subscriber at one of the extensions, as, for example, theextension. E of the private branch exchange, ringing current will betransmitted over the trunkline L, from the main oflice, therebyenergizing the left-hand winding of the relay 1. The relay 1, upon beingenergized, closes a locking circuit through its right-hand winding, andalso closes a circuit from the lamp 2, both circuits extending over thelower contact of the jack J,. The operator at the private branchexchange, upon observing the glowing of the lamp 2 inserts the plug P,into The additional key K is provided the jack J thereby opening thelower contact of the jack J and extinguishing the lamp 2. The circuit isnow extended from battery over the. sleeve conductor of the jack J andthe sleeve conductor of the plug P and thence over conductor 3 and acontact of the key K, to ground through the winding of the relay 4.Normally, battery supply is provided to the tip and ring conductors ofthe cord circuit C over the inner back contacts of the relay 4 andimpedance coils 5 and 6. hen the relay'4 is energized, however, thebattery supply connections are opened and the tip and ring conductorsleading to the plug P, are connect-- ed together through the impedancecoils 5 and 6 by a connection extending'over the inner front contact ofthe upper pair of contacts of the relay 4 and over the inner left-handback contact of the relay 7. A bridge is thus established across thecord circuit for tripping the machine ringing apparatus "at the mainoiiice in a manner well understood in the art. The relay 4 also operatesto short circuit at its lower contact the winding of the supervisoryrelay 8, which is connected in the ring conductor of the front cord.

The operator now throws the key K in order to connect her set with thecord. When the key K is in normal condition, the tip conductors of thefront and back cords are connected by a path extending from the tipcontactof the plug P, over the upper contact of the ringing key K theupper normal contact of the relay 81, contact 9 ofkey IQ, .conductor 10,contacts 11 and 12 of the key K conductor 13, contact 14 of key K, overconductor 15, upper contacts of the ringing key K to the tip contact ofplug P In a similar manner, a path extends from the ring contact of theplug P, over the lower contact of the ringing key K lower normal contactof the relay 81, contact 16 of the key K over conductor 17,;contacts 18and, 19 of the key K over conductor 20, lower contact of the relay 4,over conductor 21, contact 22 of the key K conductor 23, through thewinding of the supervisory re lay 24 associated with the back cord, overconductor 25 and the lower contacts of ring ing key K, to the ringcontact of the plug P When the key K is actuated, however. the circuitabove described is changed so that the'tip conductors of the front andback cords are connected over a path from conductor 10, over contacts 11and 26 of the key K over conductor 27, upper normal contact of relay 28,of the operators set, conductor 29, contact 30 of the key K con-' ductor31, and thence over the conductor 13 and over the path previously tracedto the tip contact of the plug P The ring connections, on the otherhand, extend from conductor 17, over contacts 18 and 32 of the key Kover conductor 33, lower normal contacts of relay 28 of the operatorsset, conductor 3%, contact 35 of the key K over conductor 20 and thenceover the path. previously traced to the ring contactof the plug P Theoperators talking and listening set comprising transmitter and receiver,T and B respectively, and the associated transformers, is bridged acrossthe circuit just described at the points'35 and 36 by means ofconductors 3'7 and 38 so that the operator is enabled to converse withthe calling subscriber or operator at the distant central exchange todetermine what extension connection is desired.

The operator, upon obtaining this information and ascertaining thataconnection to the extension E is desired, restores the listening key Kto normal and inserts the plug P, in the jack J thereby closing acircuit from ground over the sleeve contacts of the jack J and the plugP and over the conductor 39, back contact of the relay 2% and throughthe lamp 40 to battery. The lamp L0 is now caused to glow. Theoperatorthen actuates the ringing key K, to transmit ringing current tothe desired sul'isrriber over the extension E Upon the response of thesubscriber at the station associated with the extension E a supervisorycircuit is completed from the subscribers station through the cordcircuit at the private branch exchange and over the trunk line leadingto the main central o'tlice. This circuit may be traced as follows: fromone side of the battery (not shown) at the main central office over thetip conductor of the main line L,, and tip contacts of the jack J andplug P upper contact of ringing key K upper normal contact of the relay81, contact 9 of key K conductor 10, contacts 11 and 12 of key Kconductor 13, contact 1 1 of key K conductor 15, upper contact ofringing key K tip contacts of the plug P and the jack J over the tipconductor of the extension through the bridge closed by the switchhookat the subscribers station, back over the ring conductor of theextension E contacts of jack J and plug P lower contact of ringing key Kconductor 25, winding of supervisory relay 24, conductor 23, contact 22of the key K conductor 21, lower front contact of relay 4, conductor 20,contacts 19 and 18 of the key K conductor 17, contact 16 of the key Klower normal contact of relay 81, lower contact of ringing key K ringcontacts of the plug P and the jack J,, and over the ring conductor ofthe main line L, to the other terminal of the battery at the maincentral otlice. As is wellknown the circuit over the line L,, if saidline leads to a manual oliice, will include a supervisory relay in thecord circuit at. the main central office. In case the main centraloiiice is a machine switching otlice, a relay corresponding somewhat infunction to the supervising relay will beinclu'ded in the machineswitching circuit. (onsequently both the supervisory relay 2 1, which isenergized over the circuit above traced, and the supervisory relay atthe main. oitice will be under the control of the switchhook at thesubscribers station or" the extension E so that through supervision ismaintained under the control of the subscriber.

The supervisory relay 24:, upon being energized, opens the circuit ofthe lamp L0 and closes the circuit through the relay 7 from ground overthe sleeve contacts of the jack J and the plug P over conductor 39,front contact of relay 2i, winding of relay 7, and inner lower frontcontact of relay 1 to battery. The relay 7, upon being energized islocked up over its inner right-hand contact, and the contact 41 of thekey K At its outer right-hand contact relay 7 prepares the circuit ofthe relay 81, and at its outer left-hand contact short-circuits aportion of the non inductive resistance shunted about the supervisoryrelay 241-. At its inner left-hand cont-act, relay '7 opens the directcurrent bridge through the impedance coils 5 and 6, which, it will beremembered, functions to trip machine ringing at the main centraloiiice. By the opening of this bridge the shunting effect of the batterysupply from the main central office to the subsc:ibers instrument on theextension E is obviated. When the subscriber at the extension E, hangsup his receiver, the supervisory circuit previously traced is opened,and consequently the supervisory relay 24 is deenergized. If the mainline L leads to an automatic exchange, the switching machinery by whichthe connection was set up at the main, otlice will be released in awell-known manner. It, on the other hand, the main exchange at which theline L terminates is a manual exchange, the opening of the supervisor;circuit above traced results in the deonergization of a supervisoryrelay in the cord circuit at the main central ofiice, so that theoperator at such oifice will be informed that the connection is to betaken down.

The relay 24%, upon being deenergized, completes a circuit from thegrounded sleeve wire 39, over the back contact of said relay and throughthe supervisory lamp 40,

causing said lamp to glow to inform the private branch operator that theconnection is to be taken down. Another circuit is completed at the sametime from the grounded sleeve wire over the back con tact of relay 2 Lo'uter right-hand trout V I- V I contact of relay 1, and throughthewinding of relay 81 to battery. Relay 81 is now energized to disconnectthe back cord llU liU

42 is bridged across the front cord between the point of disconnectionand the plug P so that this relay? will respond to any ringing signal ofthis character, and will cause the lamp d3 to glow, thereby giving theprivate branch exchange operator a recall signal.

The relay 81, at the same time that it opened the cord circuit asalready described, connects ground to the tip conductor and battery tothe ring conductor of the end of the cord circuit associated with theplug 17,. Consequently, if the subscriber on the extension E shouldrecall by taking the receiver off the hook before the connection hasbeen taken down, a circuit is provided for energizing the supervisoryrelay 24, and by flashing the switchhook the lamp 40 may be caused toglow intermittently to attract the attention of the operator. Thesupervisory relay 24;, when again energized by the subscriber removinghis receiver before the private branch exchange operator takes down theconnection, opens the circuit of the relay 81. Relay 81 re leases andagain closes the cord circuit so that the connection extends directly tothe main central oiiice, and battery for the supervisory relay 9A is nowsupplied therefrom. With the circuit in this condition the subscribermay now extend one or more new calls through the distant central oflicewithout the aid or intervention of the private branch operator.

If no recall signal as above described should be transmitted, theprivate branch exchange operator, upon observing the glowin of the lamp40 in response to the hanging up of the receiver associated with theextension E will withdraw the plugs P, and P, from the correspondingjacks. By withdrawing the plug P the supervisory lamp 40 will beextinguished and the relay 8 will be deenergized, so that the tall:- ingconductors of the cord circuit will be restored to their normalcondition. W Vhen the plug P, is withdrawn, battery is disconnected fromthe relay 4, which is accordingly deenergized to again bridge thebattery supply across the talking conductors, and to open the circuit ofthe relay 7, thereby restoring the cord circuit apparatus to normal.

Should a subscriber on an extension such as E desire to be connected toa subscriber at the main central oflice, the operation is as followsWhen the subscriber removes his receiver from the switchhook the linerelay i-i is energized to close the circuit of the line lamp 45. Theoperator, in response to the glowing of the line lamp, will insert theplug P of the cord circuit C in the jack J thereby disconnecting theline relay 4-1 and eff'acing the line lamp. As the relay a is not nowenergized, the battery supply circuit may be traced from battery overthe inner lower back contact of the relay 4i through the impedance coil6, over contact 22 of the ke K conductor 23, through the winding of thesupervisory relay 24, conductor 25, lower contact of the key K ringcontacts of the plug P and jack J ring conductor of the extension Ethrough the subscribers bridge and back over the tip conductor of theextension E tip contacts of jack J and plug P upper contact of ringingkey K conductor 15,

contact 14 of ringing key K,, through impedance coil 5, and over theupper inner back contact of relay l to ground. The supervisory relay 24is energized over the circuit above traced and opens the circuit of thesupervisory lamp at), which would otherwise be closed over the sleevecontacts of plug P and jack J The operator now throws the key K therebyconnecting her set in the cord circuit in the manner already described,so that she is enabled to converse with the calling subscriber. Uponascertaining that the subscriber desires a connection through the maincentral office, the operator will insert the plug P, in the jack J,. Acircuit is now completed over the sleeve conductor 3 to energize therelay 4, which functions in the manner previously described todisconnect the battery supply through the coils 5 and 6, so that thebattery current for the subscribers transmitter will now be supplieddirectly from the main central office. Relay 4 also short circuits thewind ing of the supervisory relay 8, and at its inner lower frontcontact connects battery to one terminal of the winding of the relay 7,the other terminal of which is connected to ground over the contact ofthe supervisory relay 24c and the grounded sleeve conductor 39. Relay 7is accordingly energized with results previously described.

Further action upon the part of the private branch exchange operatorwill now depend upon whether or not the main central office is a manualoffice or an automatic office and Whether in the latter case the callingsubscriber at the private branch exchange desires to dial the calledsubscriber himself, or whether the calling subscriber desires theoperator to perform the dialing operation. In case the main centraloflice is a manual oflice the line relay (not shown) associated with theline L at the main. office til) will be energized as soon as the plug P,is inserted in the jack J, by means of the through supervisory circuitalready traced, if the listening key K, is in normal position, but ifthe key K, is operated as would normally be the case under theconditions now being described, a circuit for said line relay would becompleted as follows: from battery through said line relay (not shown)over the ring conductor of the line L,, ring contacts of the jack J andplug P lower normal contact of relay 81, contact 16 of key K conductor17, contacts 18 and 32 of key 11,, conductor 33, point 86, through theimpedance bridge 46 in the operators set, right-hand back contact ofrelay 47 in the operators set, over conductor 37, point 35, conductor27, contacts 26 and 11 of the key K conductor 10, contact 9 of the key Kupper normal contact of relay 81, upper normal contact of key K over tipcontacts of the plug P, of the jack J andover the tip'conductor of theline L, to ground at the main central office. The operator at thedistant central ofiice being apprised through a line lamp that a callhas come in over the line L,, answers the call in the usual manner, andthe private branch. op erator, through her telephone set which isconnected with the cord C over the contacts of the key K is able toconverse with the operator at the main office and communicate to saidoperator the desired number, so that the operation at the main officemay complete the connection. The private branch exchange operator, afterhaving passed the call to the main office operator, will restore thelistening key K, to normal, so that the talking circuit will be the sameas previously described for a call incoming from the main centraloflice.

In case the main office is a inacl'iine switching office and thesubscriber desires to dial the connection for himself, the privatebranch operator, after inserting the plug P, in the jack 3,, willrestore the key K, to normal and actuate the through dialing key K,.When the through dialing key K, actuated, the two ends of the cordcircuit are directly connected together inclependently of the variousauxiliary apparatus already described, except that the supervisory relay24: will still be included in the through dialing circuit, which is asfollows: from the tipconductor of the line L tip contacts of the jack Jand plug P upper contact-of ringing key K upper nornial contact of relay8, contact 4-8 of key B1,, conductor 15, upper contact of ringing.

key K tip contacts of the plug P and jack J,, tip conductor of theextension- E,-, tlurough the subscribefis setincluding his dial, backover the ring conductor of the extension E,, ring conductors of the plugP and jack J lower norm'al contact of ring.-

of course, be interrupted intermittently by the dialing operation, andmay cause a fluttering of the line lamp 40. This will be of nosignificance, however, except in so far as it may be an indication toapprise the operator that a dialing operation is proceeding.

hen the calling subscriber hangs up at the end of the conversation, thethrough dialing circuit which has just been traced, and which functionsas the talking circuit, is interrupted, thereby deenergizing thesupervisory relay 24:, so that the supervisorylamp 40 is caused to glow.The interruption of this circuit also causes the restoration of theswitching machinery at the distant central office. The operator, uponobserving the glowing of the lamp 40, will withdraw the plugs P, and P,and restore the apparatus to normal. It will be observed that, owing tothe fact that the ac t-uation of the key K, opened the circuit of therelay 4, relay 7 was not energized, since the circuit of the relay 7depends on the relay 4. Consequently, the feature of opening the cordcircuit through the action of the relay 8 does not come into play whenthe key K, is actuated.

In case the calling subscriber desires that the operation of dialingshall be performed by the private branch exchange operator, the operatorhaving already inserted the plug P in the ack J 2 and thrown thelistening key K, to determine the number of the called party (the key K,being at normal) will insert the plug P, in the jack J, as before. Theoperator now proceeds to manipulate the dial D in a well-known manner,to set up the connection by means of the switches at the main office. Assoon as the dial D is moved off normal, the dial switch 50 is closed,and completes the circuit for a slow release relay 51, which in turncompletes the circuit of the relay 4C7; Relay L7, at itsleft-handcontact, completes a circuit for the relay 28, which, uponoperating, opens the conductive connection which had previously existedbetween the front and back conductors of the cordthrough the operatorsset. Battery supply is now furnished to the extension E, through thewindings of the relay 52 as follows; from battery through the lowerwinding of the relay 52,- lower left-hand coil of trans i the upperfront contact of the relay 28, conductor 55, upper left-hand winding ofthe transformer 53, and through the upper I hanging up of the callingsubscribers receiver causes the release of the supervisory winding ofrelay 52 to ground. Supervisory relay 24L is held energized over thiscircuit during the dial operation, and the relay 52 is now energized.Relay 52, at its front contact, completes a holding circuit for therelay 28 to maintain the connection thus established, independently ofthe relay 4?, whose energization depends upon the dial vbeing offnormal.

lVhen relay 17 was energized by moving the dial off normal 21 dialingcircuit is completed to the trunk line L from the tip conductor of saidline over the tip contacts of the jack J and the plug P over the uppernormal contact of ringing key K upper normal contact of relay 81,contact 9 of key K conductor 10, contacts 11 and 26 of key K conductor27, point 35, conductor 37, right-hand front contact of relay 47, makeand break contacts 56 of the dial D, conductor 38, point 36, conductor33, con tacts 32 and 18 of key K conductor 17, contact 16 of key K lowernormal contact of relay 81, lower normal contact of ringing key K andring contacts of plug P and jack J, to the ring conductor of the line LThe dial operates in a well-known manner to interrupt the dialingcircuit just traced and operate the switching machinery at the mainoffice.

This dialing circuit, it will be noted, is independent of thesupervisory circuit controlled over the extension E However, at theconclusion of the dialing operation, if the operator for any reasonshould fail to restore the listening key K a holding circuit for thetrunk line L will be provided through the impedance coil 46 in theoperators set, as this coil will be bridged across conductors 37 and 38which are connected to the tip and ring conductors of the line L,respectively. This bridge is closed by the deenergization of the slowrelease relay 51, which takes place a moment after the dialing operationceases, and which results in the deenergization of the relay 17. Withthe circuit in this condition, the calling subscriber is able toconverse with the called subscriber, even though the listening key K,has not been restored, by reason of the fact that the right-hand windingof the induction coil 53 in the operators set is bridged acrossconductors 37 and 38 in parallel with the impedance coil 4C6, as soon asthe slow release relay- 51 is releasedhe transformer 53 serves toinductively couple the front and back conductors of the cord circuitthrough the operators set, so that the subscribers can talk through thetransformer. It will be noted that the circuit of the right-hand windingof the transformer 53 was held open during the dialing operation by therelay 51 to prevent the dialing impulses from being transmitted to thecalling subscriber.

At the completion of the conversation the relay 2%, which in turn causesthe energizetion of the relay 81 in a manner previously described. Relay81 opens the cord circuit as before with the result that the holdingcircuit for the line L, through the impedance coil i6 is open, and theconnection at the main central oflice is released in a wellkn'ownmanner. The connection of the battery and ground to the tip and ringsides of the cord circuit by the relay S1 is without effect at thistime, as it merely causes a flow of direct current through impedancecoil 46 without producing any useful effect. The operation of recallingeither over the line L, or the extension E in case of a delay in takingdown the connection will be as alreadydescribed, except that in thisinstance the supervisory relay 2% is supplied with direct currentthrough the windings of the relay 52 in the operators set, instead ofover the contacts of the relay 81 in the cord circuit.

If, at the conclusion of the dialing operation, the operator hadrestored the listening key K as would normally be the case, the cordcircuit would be switched back to the through supervisory conditiondescribed in connection with the call originating at the distant centraloffice, and the relay 52 would be at once released, causing the releaseof the relay 28, thereby restoring to normal of the operators set. Withthe cord circuit in the through supervisory condition, the callingsubscriber, upon hanging up, would directly cause the release of theswitches at the main office and the release of the super- -visory relay25 would give the disconnect signal and operate the relay 81 todisconnect the cord circuit as already described in connection with thecall originating at the main otlice.

In case a subscriber on the extension E,, for example, should desire tobe connected with another extension E, on the same private branchexchange-,the operator will insert the plug P in the jack J and throwthe listening key K as before. Relay 1 of the cord circuit not beingoperated battery is supplied through the impedance windings 5 and 6 tosupply current for the calling sub scribers transmitter, and to operatethe supervising relay 24:. The operator, upon ascertaining the desiredsubscriber, will insort the plug P in the jack J,. The sleeve contact oithe jack J being grounded, the relay 4 is not energized over the sleevecircuit as in the cases previously described, and consequently thebattery supply current for the called subscriber flows through theimpedance coils 5 and 6. Relay 7 is not energized by the supervisoryrelay 2% during a connection of this character for the reason that thecircuit of relay 7 depends upon the energization of the relay l. Thelamp 453 now functions as a supervisory lamp for the calling end of thecord circuit, the circuit of said lamp extending from battery throughthe lamp, over the back left-hand contact of the supervisory relay 81 inthe calling end of the cord circuit, over contact 57 of the key K overthe upper back contact of the relay l to the grounded sleeve conductor39 of the answering end of the cord circuit. hen the called subscriberon the extension E responds, the supervisory relay 81, which is includedin the battery supply circuit to the called subscriber and which is notin this instance short-circuited over the lower contact of the relay 4,becomes ener gized and opens the circuit of the supervisory lamp 43.When the two subscribers hang up, the supervisory relays 2d and 8, whichare separately controlled by the two subscribers, become deenergized andthe lamps 4:0 and 43 are again caused to glow to give the disconnectsignal. While, as above described, the end of: the cord terminating inthe plug P was used as the answering end and the end 01": the cordterminating in the plug P, as the calling end of the cord, it willfunction equally well for local connections it the plug P, be used asthe answering plug and the plug P, as the calling plug, since the twoends oi the cord are symmetrical so far as local connections areconceined. For local connections also it will be noted. that the Featureoil opening the cord circuit by means of the relay 81 in response to adisconnect signal. is not present.

The cord circuit is also adapted for use as a twitching cord to connectthe trunk line L, to some particular extension as, for example, theextension E, at night or during other perimils when no operator is onduty at the private branch exchange, thus enabling the attendant at thesubscribers station associated with the extension to answer callsincoming to the private branch exchange. When it is desired to thus usethe cord circuit, the through dialing key K, is operated and the plugsP, and P, inserted in thejacks J, and J respectively. The two ends ofthe cord circuit are now directly connected independently of theapparatus associated with the listening key K, and the apparatuscontrolled by the relays l, 7 and 81 exactly as described with referenceto the through dialing condition. While the winding of the supervisoryrelay 9A is included in the sleeve conductor, its control over the lamp520 may be prevented and wastage of current through said lamp may beobviated by opening a switch 58. The effect is that of directly connecting the tip and ring conductors of the trunk line L, with the tipand ring conductors 01 the extension line E so that the trunk line andextension line together tunction exactly like an ordinary subscribersline terminating at the main oilice.

It will be obvious that the general prin ciples herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. In a telephone system, a private branch exchange having extensionsleading to a plurality of subscribers, a trunk line leading from theprivate branch exchange to the main exchange, a cord circuit for establishing connections at the private branch exchange, a supervisoryrelay for said cord circuit, and means whereby, when a cord circuit isused in establishing a connection bet-ween said trunk line andanextension, a circuit will be established for controlling saidsupervisory relay, and a supervisory circuit controlled by thesubscribers switch hook will also be extended over said trunk line.

2. In a telephone system, a private branch exchange, a plurality ofextensions leading from said exchange, a trunk line leading from. saidexchange to a main central oliice, a cord circuit at the private branchexchange for COHlPlOiflllg' connections between extensions or between anextension and said trunl: line, a supervisory relay associated with saidcord circuit, means for supply i battery current to said cord circuit atthe private branch exchange for ei'iergizing the subscribefistelephonesets when said cord.

circuit is used in connecting local exte'n sion and said trunk line, andmeans whereby, when said cord circuit is used in establishing aconnection between an exten sion and said trunk line and a subscriberhangs up, the talking conductors of the cord iii) liil) circuit will beopened, said talking conductors, remaining closed when said cord is usedto interconnect two extensions.

e. In a telephone 7 system, a private branch exchange, extensionsleading therefrom, a trunk line extending from said exchange to a maincentral ofiice, a cord circuit at said private branch exchange "forcompleting connections between an extension and said trunk line, asupervisory relay associated with said cord circuit, means whereby, whenthe cord circuit is associated with an extension only, battery supplycurrent will be furnished locally to energize said supervisory relayunder the control of said extension, means whereby, when a connection iscompleted from said extension. through the cord circuit to the trunkline, said local battery supply is disconnected and the circuit of thesupervisory relay is extended over the trunk line, and meansresponsiveto the hanging up of the receiver by a subscriber to open saidcord clrcuit to prevent ringing current transmitted over said trunk linefrom being transmitted to the extension.

5. In a telephone system, a private branch exchange, extensions leadingtherefrom, a trunk line extending from said exchange to a main centralofice, a cord circuit at said private branch exchange for completingconnections between an extensionand said trunk line, a supervisory relayassociated with said cord circult, means whereby,

when the cord circuit is associated with an extension only, batterysupply current will be furnished locally to energize said supervisoryrelay under the control of said extension, means whereby, when aconnection is completed from said extension through the. cord circuit tothe trunk line, said local. battery supply is disconnected and thecircuit of the supervisory relay is extended over the trunk line, andmeans responsive to the hanging up of the receiver by a subscriber toopen said cord circuit to prevent ringing current transmitted over saidtrunk line from being transmitted to the exten sion, said means alsooperating to supply current to the portion of the cord circuitassociated with the extension, in order that the supervisory relay maybe operated for recall purposes.

6. In a telephone system, a private branch exchange, extensions leadingtherefrom, a trunk line extending from said exchange-to a main centraloflice, a cord circuit atsaid private branch exchange for C0111- pletingconnections between an extension andsaid trunk line, a supervisory relayassociated with said cord circuit, means whereby,vwhen.the cord circuitisassociated with an extension only, battery supply current will befurnished locally to' energize said supervisory relay under the controlof said extension, means whereby, when a connection is completed fromsaid extension through the cord circuit to the trunk line, said localbattery supply is disconnected and the circuit of the supervisory relayis extended over the trunk line, and means responsive to the hanging upof the receiver by a subscriber to open said cord circuit to preventringing current transmitted over said trunk line from being transmittedto the extension, said means also operating to supply current to theportion of? the'cord circuit associated with the extension, in orderthat the supervisory relay may be operated for recall purposes, andmeans responsive to ringing current and associated with the end of thecord circuit connected to the trunk line for producing a recall signalwhen ringing current-is transmitted from the main central office.

7. In a telephone system, a private branch exchange with extensionsleading therefrom, a trunk line extending from said exchange to a main,central ofiice, a cord circuit for establishing connections between saidextensions and. said trunk line, a supervisory relay for said cordcircuit, switching means associated with said cord circuit, and circuitscontrolled by said switching means whereby when the cord circuit is usedin establishing a connection between an extension and said trunk line,dial signals for operating mechanical switches at the main centralofiice may be transmitted from said extension line directly through saidcord circuit and supervisory relay to the trunk line.

8. In a telephone system, a private branch exchange, extensions leadingtherefrom, a trunk line extending from said exchange to a main centraloflice, a cord circuit at said exchange for completing connectionsbetween extensions and said main line, an operators set includingdialing means adapted to be associated with said cord circuit, andswitching means whereby, when said cord circuit is used to interconnectan extension and said main line, the dialing means of the operators setmay, under certain conditions, be used for transmitting signals throughthe cord circuit and over the main line to actuate mechanical switchesatthe main ofiice, and whereby, under certain other conditions, dialsignals for setting said; mechanical switches may be transmittedtronithe extension through said cord circuit to the mainline.

9. In a telephone system, a private branch exchange, extensions leadingtherefrom, a trunk line extending to a main central ofice, a cordcircuit for completing connections between extensions and between anextension and said trunk line, a supervisory relay for said cordcircuit, meanst'or interconnecting the ends otsa'id cord circuit throughauxiliary apparatus including means for controlling the supervisoryarrangements of the cord circuit and the operators set, and switchingmeans whereby the ends 01 the cord circuit may be directly connectedthrough said supervisory relay and independent of said auxiliaryapparatus, so that the cord may be used for patching an extension tosaid trunk line, whereby the extension becomes in effect a continuationof the trunk line.

10. In a telephone system, a private branch exchange, extensions leadingtherefrom, a trunk line extending from said exchange to a main centralofiice, a cord cir' cuit at said private branch exchange for completingconnections between an extension and said trunk line, a supervisoryrelay associated with said cord circuit, means whereby, when the cordcircuit is associated with an extension only, battery supply currentwill be furnished locally to energize said supervisory relay under thecontrol of said extension, means whereby, when a con nection iscompleted from said extension through the cord circuit to the trunkline, said local battery supply is disconnected and the circuit of thesupervisory relay is extended over the trunk line, and means responsiveto the hanging up of the receiver by a subscriber to open said cordcircuit to prevent ringing current transmitted over said trunk line frombeing transmitted to the extension, said means also operating to supplycurrent to the portion of the cord circuit associated with theextension, whereby the supervisory relay may be operated when the switchhook is again actuated and means whereby when the supervisory relay isthus operated the cord circuit is again closed and battery is suppliedfrom the main central office to maintain said supervisory relayoperated.

In testimony whereof, I have signed my I name to this specification this22nd day of October, 1920.

vRAND S. BAILEY.

